Abstract
A detailed morphological staging system for cattle embryos at stages following blastocyst hatching and preceding gastrulation is presented here together with spatiotemporal mapping of gene expression for BMP4, BRACHYURY, CERBERUS1 (CER1), CRIPTO, EOMESODERMIN, FURIN and NODAL. Five stages are defined based on distinct developmental events. The first of these is the differentiation of the visceral hypoblast underlying the epiblast, from the parietal hypoblast underlying the mural trophoblast. The second concerns the formation of an asymmetrically positioned, morphologically recognisable region within the visceral hypoblast that is marked by the presence of CER1 and absence of BMP4 expression. We have termed this the anterior visceral hypoblast or AVH. Intra-epiblast cavity formation and the disappearance of the polar trophoblast overlying the epiblast (Rauber’s layer) have been mapped in relation to AVH formation. The third chronological event involves the transition of the epiblast into the embryonic ectoderm with concomitant onset of posterior NODAL, EOMES and BRACHYURY expression. Lastly, gastrulation commences as the posterior medial embryonic ectoderm layer thickens to form the primitive streak and cells ingress between the embryonic ectoderm and hypoblast. At this stage a novel domain of CER1 expression is seen whereas the AVH disappears. Comparison with the mouse reveals that while gene expression patterns at the onset of gastrulation are well conserved, asymmetry establishment, which relies on extraembryonic tissues such as the hypoblast and trophoblast, has diverged in terms of both gene expression and morphology.
Highlights
There are two main reasons why it is important to better understand the early developmental events in cattle
Is a high correlation of embryonic age with two of the measures of embryonic stage, namely embryo size and epiblast length, there is a huge range of sizes at any given day of embryo age
We present here a novel staging system for cattle embryos between hypoblast formation and the start of gastrulation
Summary
There are two main reasons why it is important to better understand the early developmental events in cattle. Expression was analysed for genes whose homologues mark distinct tissues in the early the mouse embryo These were the polar trophoblast markers Furin [24], Bmp4 [25] and Eomesodermin (Eomes) [26], the epiblast markers Nodal [27] and Cripto (Tdgf1) [28], the anterior hypoblast markers Cerberus (Cer1) [29] and the prospective mesoderm markers Brachyury (T) [30] and Eomes [31]. Cer is expressed in anterior mesendoderm cells and their progenitors in the primitive streak [29, 32] We chose these seven marker genes as they have all been functionally implicated in the patterning of the mouse embryo and were deemed most informative for understanding the mechanisms driving early development in cattle
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